A traditional method of forming inorganic oxide glass requires melting of glass forming compounds, such as SiO.sub.2, at high temperatures (e.g. greater than 1400.degree. C.). A more recent method has been described for glass preparation which does not require such high temperatures. Using this method, known as the "sol-gel" method, the condensation of reactive metal oxide monomers can occur in the liquid phase at temperatures in the range of 25.degree.-60.degree. C. The sol-gel reaction is a two-step process during which metal alkoxides are hydrolyzed to form metal hydroxides, which in turn condense to form a three-dimensional network. The sol-gel products of inorganic components are generally sintered to produce hard and brittle glass. ##STR1##
The sol-gel method allows the formation of hybrid composite materials made of inorganic (glass) and organic components which would not survive the very high temperatures of traditional glass making methods. Such a composite material can provide advantages resulting from the combination of the tensile strength and impact resistance of the organic polymer and the compressive strength of the inorganic matrix. The introduction of organic groups into glass can thus provide variations in properties such as strength, toughness, stiffness, brittleness, hardness, homogeneity, density, free volume, and thermal stability. Secondary considerations include resistance to corrosion, creep, and moisture. Both the strength and stiffness of a composite can be derived from the properties of the reinforcing fiber. Toughness results from the interaction between the matrix and the fibers. Such composite materials may be used in the manufacture of piezoelectric, ferroelectric, electro-optic, and superconducting fibers and films (Schmidt, H. 1989 J. Non-Crys. Sol. 112, 419-423; Ulrich, D. R. 1990 J. Non-Crys. Sol. 121, 465-479).
U.S. Pat. No. 4,584,365 is directed to formation of a polymer of metal alkoxide wherein polymerization occurs between the SiO.sub.2 groups. The alkoxide groups are not polymerized. Huang, H. H. et al (1987, Macromolecules 1987, 20, 1322-1330) reported the formation of a similar composite. ##STR2##
To further control and vary the properties of the composite, it is desirable to incorporate polymerized organic groups within the glass network. Phillip, G. et al (1984 J. Non-Crys. Sol. 63, 283-292) reported the synthesis of a silaceous network cross-linked by chains of covalently bonded polymethacrylates. This method involved copolymerization of epoxysilane, methacryloxysilane, and a titanium tetralkoxide to form a flexible silaceous network suitable for use in contact lenses. This product was reported to have low shrinkage during curing (Schmidt, H. 1989 supra). ##STR3##
Wei, Y. et al (1990 Chemistry of Materials 2, 337-339) reported the synthesis of composite materials by co-condensation of tetraethoxysilane with acrylate polymers containing triethoxysilyl groups. The resulting product was composed of covalently bonded organic and silicic groups. ##STR4##
Wilkes, G. L. et al (1990 J, Am. Chem. Soc.) reported the incorporation of poly(dimetheylsiloxane) (PDMS) oligomers into a covalently bonded network with tetraethyloxysilane. The resulting material was inhomogeneous, with localized phase separation of the PDMS component. ##STR5##
Polymer-modified glasses similar to the above structure based on silicon, titanium, and zirconium were synthesized by Mark, J. E. et al (1987 Polymer Bulletin 18, 259-264) and Glaser, R. H. et al. (1988 Polymer Bulletin 19, 51-57).
In order to further control the properties of composites, is considered desirable to obtain composite materials in which the inorganic glass network and the organic polymer are interpenetrating, but not covalently bonded to each other. A silica gel-polymer composite was produced by impregnating a pre-prepared silica gel with methyl methacrylate monomer, then catalyzing the polymerization in situ of the methacrylate to polymethacrylate (Pope, E. J. A. et al 1989 J. Mater. Res. 4, 1018-1026). This method is limited by the necessity to preform the silica gel and by the requirement for homogeneous penetration of the monomeric species into the gel in order to produce a homogeneous composite.